Transmission of information via light is a known technique. For ages we have been using infrared light to control devices such as televisions and audio/video systems in our house. However, as light sources emitting visible light are typically all around us, for example, in our houses and offices, ideas have evolved to transmit information using our ambient light sources. Especially challenging when using visible light is to enable data transmission without unwanted visible effects such as flicker, because the human eye is quite sensitive to intensity variations.
Since the introduction of solid state light sources in offices and houses this data transmission using the ambient light has re-emerged. Solid state light sources have several parameters that can be varied and controlled relatively easily. Such parameters include light intensity, light color, light color temperature and even light direction. For data transmission, relatively high switching frequency of the light source is important. So data may be embedded in light by modulating it. The simplest form of modulation is on-off switching at a certain frequency. This modulation is invisible for humans if this frequency is large enough, typically larger than 100 Hz. For most applications we may need to transmit more data than just the value of the embedded frequency. Different modulation methods may be used and one of these modulation method may be a technique known as frequency shift keying (further also indicated as frequency shift key) in which the frequency of the modulation is changed over time. However, early experiments showed that although the frequencies used in frequency shift key were well above 100 Hz, still visible flicker is perceived.